Fume hoods have long been used in research laboratories and the like to protect laboratory personnel from chemical and biological hazards which are attendant with the research environment. Although many different fume hood designs exist, a fume hood, generally speaking, includes a table-like structure having a flat working surface upon which is positioned a substantially rectangular shaped enclosure. This rectangular enclosure is defined by a roof member, a floor, a left wall, a right wall, a front wall, and a back wall. The front wall of the enclosure is defined, in part, by an access opening, or aperture, and a fume hood sash is movably borne by the fume hood and is operable for substantially vertical movement. This fume hood sash is often manufactured of a thick, heavy glass panel which may be located in a closed position, or alternatively may be selectively located to occlude the aperture thereby providing any desired amount of access to the protected working environment which is provided by that enclosure.
Fume hoods perform a number of laudatory functions, however their most important functional aspects are to provide a safe working surface to protect laboratory personnel and others, from the effects of harmful chemical splashes and spills or exposure to biological agents, and to evacuate noxious vapors and other gases or agents from the immediate working environment. Therefore, it is desirable that a fume hood have a means for selectively varying the size of its associated access opening or aperture to meet the demands of the laboratory environment. Heretofore, previous laboratory fume hoods have incorporated weighted counterbalance assemblies to provide a means for controlling the size of the access opening. Such counterbalance assemblies are operable to selectively retain the fume hood sash in any desired position and also to facilitate the raising and lowering of the fume hood sash, which is often quite heavy, within the supporting framework which defines the fume hood access opening.
A representative prior art, fume hood sash counterbalance assembly generally includes two separate and independently mounted sash counterbalance weights which are concealed within the opposite front corners of the enclosure and which are secured on the fume hood sash by two lifting cables. Although this particular sash counterbalance assembly design has operated with varying degrees of success, it is subject to shortcomings which have detracted from its usefulness. For example, in such a sash counterbalance assembly, the two separate counterbalance weights are impeded in their movement when they individually bump and scrape the inside surfaces of the enclosure structure when the fume hood sash is either raised or lowered. This bumping and scraping produces assorted annoying sounds, and creates friction between the counterbalance weights and the enclosure structure. This, of course, results in rough and jerky movement of the fume hood sash as it moves along its path of travel by virtue of the fume hood sash having to overcome the effects of this same friction to continue its vertical movement.
Further, this type of fume hood counterbalance assembly does not provide a means for preventing the fume hood sash from becoming cocked, lodged or otherwise misaligned within the access opening during the vertical movement of same. This latter shortcoming of known counterbalance assemblies is particularly troublesome and dangerous to personnel handling a hazardous substance while utilizing a fume hood during biological and chemical experimentation. In the past, and on those occasions when a fume hood sash became misaligned or lodged during use of the associated fume hood, laboratory personnel may have employed various expedient means to address this problem. For example, and if such an event was to occur and a laboratory worker was utilizing both hands to maneuver a hazardous substance, this same individual may have solicited assistance from surrounding personnel to dislodge the fume hood sash by applying physical force to same thereby avoiding an accidental spillage of the hazardous substance. This is, of course, undesirable inasmuch as it needlessly exposes other laboratory personnel to the hazardous environment of the fume hood.
Ideally, and to prevent a fume hood sash from becoming misaligned or lodged in a fume hood access opening, a fume hood sash should be raised and lowered in a substantially level position. This desired position may be achieved only if the left and right sides of the fume hood sash are raised or lowered in correspondingly equal amounts and at a substantially constant velocity. However, and in the prior art sash counterbalance assemblies, the two counterbalance weights have generally operated separately, and independently of each other, thereby permitting the left and right sides of the fume hood sash to travel in unequal amounts and at varying velocities. Such unequal movement by the left and right sides of the fume hood sash has often caused the sash to become cocked or lodged within the fume hood access opening, resulting in an unsafe operational condition in the immediate laboratory environment.
In recent years, other fume hood sash counterbalance assemblies have been designed to eliminate some of the existing shortcomings. For example, U.S. Pat. No. 3,934,496, to Turko, teaches the utilization of a single sash lifting cable and a single counterbalance weight to provide for smoother travel of the fume hood sash during vertical movement in the fume hood access opening. In this single weight counterbalance assembly, the ends of a single cable are connected to opposite sides of a fume hood sash and are routed over the top of the enclosure structure by pulleys. The middle of this single cable is then attached to a counterbalance weight which is located in the back of the enclosure. While this device may operate with some degree of success, that is, the counterbalance assembly may reduce the annoying sounds produced by other known counterbalance assemblies, such counterbalance assemblies do not provide a means for synchronizing the velocity of the opposite sides of a fume hood sash during vertical movement of the sash to prevent the sash from becoming misaligned during use of the fume hood by laboratory personnel.
Therefore, a need has existed for a sash counterbalance assembly which synchronizes the velocity of the opposite sides of a fume hood sash during any movement of the sash within a fume hood access opening thereby causing the sash to travel in a substantially constant level position to prevent the sash from becoming misaligned or lodged in the access opening thus increasing the hazards of the laboratory environment.